Birefringent materials are widely used in applied optics. Birefringent materials display two different indices of refraction, due to optical anisotropy. An optically anisotropic material is one in which the optical properties are not the same in all directions. Due to this anisotropy, a radiation beam directed into a birefringent medium will be split into two beams of differing angles of refraction. A radiation beam made up of two parts in differing polarisation states incident on a birefringent material will be divided; one part of the beam is refracted according to the ordinary refractive index while the second part of the beam is refracted according to the extra-ordinary refractive index.
A Wollaston prism is an example of an optical device that employs birefringent materials. A Wollaston prism is a polarizing beam splitting prism that comprises a non-birefringent part and birefringent part. A radiation beam shone through the prism is separated into two, orthogonally polarized rays at the interface between the parts. Wollaston prisms are used in microscopes, such as the Nomarski microscope, in which the orthogonally polarized rays are used to scan two different areas of a three-dimensional sample. The characteristics of a Wollaston prism are not variable, however.
It is often desirable to alter the characteristics of a lens in order to alter the direction and angle of output rays, e.g. for the purposes of focusing on a microscope sample. Liquid crystal lenses have been used in optical scanning devices for the purpose of scanning multi-layer optical storage media (‘DVD pick-up system reads two layers simultaneously’—Optics and Lasers Europe, September 2000). Liquid crystals consist of elongate molecules that are capable of flowing freely, but are also capable of interacting to form and sustain large scale order, in the manner of a crystal. Nematic liquid crystals consist of molecules that tend to lie substantially parallel. When the molecules are oriented such that they are substantially parallel, the liquid crystal is optically anisotropic, and is therefore birefringent. The direction of the parallel orientation of the liquid crystals can be controlled by providing an alignment layer which orients the molecules, and by applying a voltage. When a voltage is applied, the molecules will rotate into alignment with the field.
Controlling the properties of liquid crystal lenses for optical purposes by altering the nematic orientation of the constituent molecules by applying a voltage is not always ideal, as the crystals take some time to align in response to the voltage. The time taken to align may be too slow for the purposes of, for example, altering the shape of the lens while scanning an optical medium in real time.